Graded particulate compositions

ABSTRACT

A method of forming a particle mass comprising at least two particle populations arranged in a desired graded relationship, comprising: forming in a container a first layer of dry particles constituting a first particle population having a desired particle size distribution, superimposing on the first layer a second layer of dry particles constituting a second particle population having a desired particle size distribution, the second layer being in direct contact with the first layer at a contact interface, and causing the particle mass in the container to vibrate to cause a desired degree of migration of particles from one or both layers across the contact interface under the influence of force experienced by particles in the mass. The particle populations may have different physical and/or chemical properties, so that the particle mass is functionally graded for subsequent fusion into a functionally graded material such as a ceramic or ceramic/metal composite.

This invention relates to a method for forming a particle masscomprising at least two particle populations arranged in a desiredgraded relationship. In particular, the particle populations may havedifferent physical and/or chemical properties, so that the particle massis functionally graded for subsequent fusion into a functionally gradedmaterial such as a ceramic or ceramic/metal composite.

BACKGROUND TO THE INVENTION

Functionally graded materials have physical and/or chemical propertieswhich change in a smooth rather than abrupt or stepwise manner along avector through the material, usually its thickness, One important classof such materials consists of those which are formed by fusion of a(sometimes compacted) particle mass, such as ceramics and metal/ceramiccomposites. Functionally graded ceramics or ceramic/metal composites areuseful for a large number of applications, including wear partsgenerally; engine and pump components such as seals, gaskets and valves;cutting tools; drawing and extrusion dies; brake discs; armour; thermalbarrier coatings; liquid and gas filters; and specialist instrument andelectronic applications.

For fused particle materials, the particle mass is built up bydeposition of a thickness of particles whose composition changes duringthe build up, the changing composition reflecting the changing physicaland/or chemical properties required for the desired functional gradingof the finished material. Where the material is in the form of acoating, particle build up is sometimes by means of chemical or physicalvapour deposition onto the substrate, but these processes are slow, andthe range of materials to which they are applicable is limited. Sprayingof electrostatically charged particles, as in ink-jet printing methods,is sometimes used for both coatings and shaped articles, but theporosity of the article dr coating is often high and again the range ofmaterials suitable for the process is limited. Where the material is inthe form of a moulded article, techniques include

-   -   (i) co-fusion of a stacked tape assembly, for example        co-sintering of ceramic tapes, the properties of successive        tapes in the stack changing to reflect the functional grading        desired in the fused article. This process suffers from the        disadvantage that the functional property transition from tape        to tape is rather abrupt.    -   (ii) self-propagating high temperature synthesis of layered        powders, where a stack of thin layers of particles which are        exothermically reactive is caused to fuse by the heat of the        reaction, fusing first at the layer interfaces and subsequently        in the interior of each layer. Here also the composition of the        layers is selected to reflect the functional grading desired in        the fused article, but the process suffers from the        disadvantages that only a limited choice of exothermically        reactive powders is available, and the porosity of the material        is often high.    -   (iii) graded casting, where the thickness of the material is        built up by continuously mixing a plurality of different        particle slurry compositions and feeding the resultant mixed        slurry into a porous mould. The proportions of each slurry being        mixed and then dispensed into the mould are varied during mould        filling to create the correct mix of particles for the        functional property required at that point in the thickness of        the material. Liquid is drained from the mould during filling so        that the particle mass builds up in the mould as a wet cake.        This process suffers from the disadvantages that control over        the mixing of particle slurries can be crude, liquid extraction        and wet cake drying are time consuming, typically more than 24        hours.

BRIEF DESCRIPTION OF THE INVENTION

This invention makes available an alternative method for forming aparticle mass comprising at least two particle populations arranged in adesired graded relationship, especially those where the particlepopulations have differing physical and/or chemical properties. Theprocess is based on vibration of a particle mass comprising dry particlelayers stacked one on another, the vibration being controlled to cause adesired degree of migration of particles from one layer to anotheracross layer boundaries, such migration being primarily a result of thedifferent migration rates of different particle sizes in the vibratedmass under the influence of forces applied to the particles in the mass,principally gravitational and/or centripetal force, but in some casesalso magnetic or electromagnetic forces. Being a dry process, it isconveniently quick. It is applicable to a wide range of potentiallyuseful particle materials, and allows fine control over the arrangementof particles in the resultant graded mass.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of forming a particle masscomprising at least two particle populations arranged in a desiredgraded relationship, the method comprising forming in a container afirst layer of dry particles constituting a first particle populationhaving a desired particle size distribution, superimposing on the firstlayer a second layer of dry particles constituting a second particlepopulation having a desired particle size distribution, the second layerbeing in direct contact with the first layer at a contact interface, andcausing the particle mass in the container to vibrate to cause a desireddegree of migration of particles from one or both layers across thecontact interface under the influence of force experienced by particlesin the mass, for example gravitational and/or centripetal and/ormagnetic and/or electromagnetic force.

As used herein the term “particle” refers to a small body of inorganicor organic material having a shortest dimension of from about 5 nm toabout 100 μm, and having a longest dimension no greater than about threetimes that of the shortest dimension. Within those dimensional limits,the shape of a “particle” is irrelevant, and includes generallyspheroidal, ellipsoidal, polyhedral and plate-like geometries as well asirregular shapes. “Particles” may be crystalline or amorphous, and maybe solid, hollow or otherwise cavitied.

As used herein the term “microfibre” refers to a synthetic polymer orinorganic body which has a length of from about 1 μm to about 1 mm, andwhose length is more than ten times its thickness. Often microfibreswill have a generally circular, tubular or elliptical cross sectionand/or blunt ends. Synthetic polymer microfibres may be of, for example,polyethylene, polypropylene and the like, and inorganic microfibres maybe of, for example, glass, silicon carbide, alumina, carbon, steel andthe like.

As used herein the term “whisker” refers to a body which has a length offrom about 1 μm to about 100 μm, and whose length is between three andten times its thickness. Often whiskers will have a generally circularor polyhedral cross-section and/or sharp ends. Examples of whiskermaterials include silicon carbide, alumina, boron carbide, tantalumcarbide, and niobium carbide.

As used herein the term “dry particles” refers to a particle populationwhich is free flowing under the vibrational conditions of the method ofthe invention. It does not exclude the presence of small quantities ofliquids, providing they are insufficient to prevent such free flow ofparticles.

Container

The method of the invention is carried out on particle layers in acontainer. The container may be a mould defining the desired shape ofthe particle mass, which in turn may define the desired shape of anarticle created by fusing the particles into a coherent mass. In oneembodiment, the base of the mould may be separable from its side walls,so that after the method of the invention is applied to the particlemass in the mould, a particle fusion step also fuses the mass to thebase of the mould which then forms part of the fused article. Thisembodiment may be useful, for example, where the fused particles are toform a coating on a substrate constituted by the mould base.

Friction effects between the wall of the container and the particlesmay, in some cases, affect the uniformity of particle migration acrosslayer boundaries, for example by reducing the depth of intermixingadjacent the walls relative to that remote from the walls. Where sucheffects are undesirable for any given application, they may be reducedby use of low friction materials for the container walls and in somecases by appropriate choice of container shape.

Particle Layers

The method of the invention involves forming a stack of layers of dryparticles in a container, each in contact with the layer on which it issuperimposed at a contact interface.

Particles in a layer may be of more than one particle material. Aparticle layer may include non-particle components, for examplemicrofibres and/or whiskers. Where more than one particle material ispresent in a layer, and/or where a layer contains non-particlematerials, the components of the layer are preferably pre-blended beforeforming the layer in the container from the pre-blend.

The particles of each layer are selected to have a desired particle sizerange. It is presently believed that, for a given particle populationunder vibration in a gravitational or centripetal field the distanceover which a particle migrates is largely determined by its size—i.e.smaller particles migrate over larger distances than larger particles.Other factors such as density and surface effects may also affect thedistance over which particles migrate, but such effects are likely to besmall when compared with the effect of particle size. Any microfibreand/or whisker materials in a layer are unlikely to migrate to anysignificant extent compared to particles, since their shape offersgreater resistance to movement through the particle matrix of the layer.

With the above principle in mind, the particle size ranges of twocontiguous layers can be selected, together with the vibrationconditions, to achieve a desired degree of migration of particles fromone or both layers, across the contact interface between the layers, sothat particles from one or both layers infiltrate the other layer(s) toproduce a desired graded arrangement of particles along a vector throughthe layers in the direction of the gravitational or centripetal force.Where the particles of each layer are selected to have differentphysical or chemical properties, the effect of the migration across thecontact interface between layers is to produce a functionally gradedarrangement, where the function is determined by the physical and/orchemical properties of the particles of each layer and mixtures thereofin various proportions.

Functional properties for which particle populations of the layers mightbe chosen include wear, toughness and/or reinforcement, thermal,electrical, magnetic, chemical resistance, and surface properties.

The particle materials present in any layer may be selected from a widerange of possibilities. Important classes of particle materials areceramics such as silicon carbide, alumina, silicon nitride, zirconiumoxide, boron carbide, boron nitride, and tungsten carbide; metals suchas tungsten, boron, steel, and cobalt; and synthetic resin materialssuch as epoxy, polyester, and phenol-formaldehyde resins.

Multiple Layers

The simplest particle layer stack to which the method of the inventionmay be applied consists of two particle layers. However, the method isalso applicable in cases where more than two layers are required toconstruct the eventual desired size-graded particle mass or fusedarticle. In such cases one or more additional dry particle layers is/aresuccessively stacked on the second or a subsequent layer of the stack,each additional layer (like the first and second layers) comprising dryparticles which constitute a particle population having a desiredparticle size distribution, and (again like the first and second layers)each additional layer being in direct contact at a contact interfacewith the layer on which it is superimposed, and the particle mass isvibrated after completion of, or at intervals during the assembly of thestack to cause a desired degree of migration of particles across one ormore of the contact interfaces under the influence of force such asgravitational, centripetal, magnetic or electromagnetic force. Forexample, where the intended graded particle mass is to be constructedfrom four layers of particles,

-   -   all four layers might be stacked and then vibrated; or    -   the first two layers might be assembled and vibrated and the        other two layers might then be successively stacked on the        particle mass formed by the first two layers, before vibrating        the whole assembly; or    -   the first two layers might be assembled and vibrated, the third        layer stacked on the particle mass formed by the first two        layers, then vibrated, and the fourth layer stacked on the        particle mass formed by the first three layers, then vibrated.        Vibration and Compaction

The method of the invention requires that the particle mass in thecontainer be vibrated to cause a desired degree of migration ofparticles across the contact interface(s) between layers under theinfluence of force experienced by particles in the mass, for examplegravitational and/or centripetal and/or magnetic and/or electromagneticforce. Usually the force applied to the particles will beunidirectional, along the grading vector through the mass, normally itsthickness. In most cases, the particle layers will be stacked generallyhorizontally, so that vibration takes place under the influence ofgravity. However, it is also feasible to assemble the stacked particlelayers at a substantial angle to the horizontal or even vertically, byfor example spinning or rotating the assembly as the layers are created,so that centripetal force is applied through the thickness of the stackas the stack during layer assembly and during vibration. For example,where the desired graded particle mass is to be in the form of acylinder, a first layer of particles might be formed by injecting theparticles into a cylindrical mould while spinning the mould about itslongitudinal axis. Centripetal force then spreads the particles as acylindrical layer on the interior surface of the mould. A secondparticle population might then be injected into the mould while it isstill spinning, to form a second cylindrical particle layer superimposedon, and in contact with the first. The mould might then be vibratedwhile spinning, to cause the desired particle migration across thecontact interface between layers. In cases where one or more of theparticle layers contains metal or electrically conductive particles, itmay be advantageous to influence these particles during vibration by theapplication of a magnetic or electromagnetic field to the particle mass.

Whether it takes place under gravitational, centripetal or other force,the amplitude, frequency and duration of the vibration will be tailoredto the required degree of intermingling of particles required in thedesired graded particle mass. Because the method of the invention isquite generally applicable to a wide range of particle size populations,with or without additives such as microfibres and whiskers, for theproduction of a wide range of fused articles, it is not possible tospecify universally preferable combinations of ranges for thoseparameters. Some general comments may however be made: Vibration ofparticle populations can produce two effects, increased particle packingdensity and fluidisation, which decreases packing density. In mostcases, particles introduced into a container for treatment in accordancewith the invention will not initially be packed at the highest potentialpacking density. Vibration of non-optimally packed particles canincrease the packing density. In general, for any given particlepopulation vibrated at a given frequency, a smaller vibrationalamplitude (the parameter which controls the acceleration to which theparticles are subjected) is required to increase packing density than toproduce fluidisation. Since the migration of particles across layerboundaries mainly occurs while the particles are fluidised, vibration inaccordance with the invention should preferably include a period whenthe frequency and amplitude are chosen to produce fluidisation. Thisdoes not rule out a period of vibration where the frequency andamplitude are chosen to increase packing density, either before or(especially) after the fluidisation vibration.

However, for typical ceramic or metal ceramic composites, it will oftenbe the case that for fluidisation the frequency of vibration will be inthe range 50-2000 Hz, for example 50-500 Hz, such as 75-150 Hz (about100 Hz ofteen being appropriate for particle populations with a meanparticle size of about 1 micometer), and the amplitude will be in therange 1 μm to 10 mm. The duration of vibration will often be of theorder of from a few seconds to a few minutes, in many cases, one or twominutes.

Vibration of the particle mass in the container may be indirect, byvibration of the container itself, or direct, by vibration of probesinserted into the mass to be vibrated. In very specialised instances, itmay be desirable to cause the particle mass to vibrate by placing thecontainer containing the mass in an ultrasound field.

The particle layers being treated in accordance with the invention maybe pressure compacted, during and/or after vibration. During thefluidisation phase of the vibration, light pressure compaction, forexample by means of a plate in contact with the top layer of a layerstack, may be beneficial in controlling particle dust clouds thrown offby the fluidisation. Of course, any such pressure compaction during thefluidisation vibration may increase the inter particle friction(increase locking stress between particles), making it necessary toincrease amplitude (acceleration) to achieve fluidisation andcross-boundary mixing. In general it may be more convenient to operateat lower amplitudes for safety reasons, so pressure compaction duringthe fluidisation vibration should preferably be low.

Fusion

In many cases, the method of the invention will be applied for theproduction of coherent, monolithic articles or coatings formed by fusionof the graded particle mass. Such fusion will in many cases involvesintering or melting the particles of the mass by heating. This will bethe case for ceramic, metal, or ceramic/metal composite articles.However, some articles may be fused by chemical reaction, such as bycross-linking or other chemical bonding between synthetic polymerparticles, for example in the case of liquid or gas filters formed frombonded polymer particles, whose size (and thus the filter pore size), orwhose size and adsorbency, varies along the flow path through thefilter.

1. A method of forming a particle mass comprising at least two particlepopulations arranged in a desired graded relationship, the methodcomprising: forming in a container a first layer of dry particlesconstituting a first particle population having a desired particle sizedistribution, superimposing on the first layer a second layer of dryparticles constituting a second particle population having a desiredparticle size distribution, the second layer being in direct contactwith the first layer at a contact interface, and causing the particlemass in the container to vibrate to cause a desired degree of migrationof particles from one or both layers across the contact interface underthe influence of force experienced by particles in the mass.
 2. A methodas claimed in claim 1 wherein the particle mass in the container iscaused to vibrate under the influence of gravitational force,centripetal force, magnetic electromagnetic force, or combinationsthereof.
 3. A method as claimed in claim 1 wherein the particle mass iscaused to vibrate by vibrating the container.
 4. A method as claimed inclaim 1 wherein one or more additional dry particle layers is/aresuccessively stacked on the second or a subsequent layer of the stack,each additional layer comprising dry particles constituting a particlepopulation having a desired particle size distribution, each additionallayer being in direct contact at a contact interface with the layer onwhich it is superimposed, and the particle mass is vibrated aftercompletion of, or at intervals during the assembly of the stack to causea desired degree of migration of particles across one or more of thecontact interfaces under the influence of gravitational, centripetalforce or other applied forces.
 5. A method as claimed in claim 1 whereinone or more layer of particles also contains whiskers, microfibres, orcombinations thereof.
 6. A method as claimed in claim 1 wherein at leasttwo particle layers in direct contact at a contact interface compriseparticle populations which are selected such that the population of onelayer has a desired set physical properties, chemical properties, orboth different from that of the population of the other layer.
 7. Amethod as claimed in claim 1 wherein the container containing theparticle layers is a mould defining the desired shape of the particlemass.
 8. A method as claimed in claim 1 wherein the particle mass in thecontainer is pressure compacted during or after the vibration step.
 9. Amethod as claimed in claim 1 wherein at least one particle layercomprises ceramic particles.
 10. A method as claimed in claim 1 whereinat least one particle layer comprises metal particles.
 11. A method asclaimed in claim 1 wherein at least one particle layer comprises polymerparticles.
 12. A method as claimed in claim 1 wherein a particle layercomprising ceramic particles is superimposed at a contact interface on aparticle layer comprising metal particles or vice versa.
 13. A method asclaimed in claim 1 wherein at least one particle layer is prepared bypre-blending the components thereof prior to forming the layer in thecontainer.
 14. A method as claimed in claim 1 wherein after thevibration step the particle mass is fused into a coherent article. 15.An article obtained by the method of claim 14.